The prior art includes some solutions for dimming switches for LEDs. These solutions typically utilize field effect transistors (FETs) to control the current driven through the LEDs. BJTs have not been used in this context because of two drawbacks. First, BJTs have a slow switching speed compared to FETs. Second, because the current through the base of a BJT is added to the current through the emitter of the BJT, the current through the LEDs (which would be connected to the collector of the BJT) cannot be properly sensed and controlled. However, high-voltage BJTs are cheaper and easier to manufacture than FETs, and the overall cost of a dimming switch could be improved by using BJTs instead of FETs.
A dimming switch for LEDs that utilizes a FET and BJT in a cascode configuration can combine the benefits of the fast switching speed of an FET and the low cost of a BJT. However, the source current of the low-voltage FET in a cascode configuration will not be an accurate representation of the current through the LEDs connected to the BJT collector, as the source current of the FET will be offset by base current of the high-voltage BJT. Therefore, a current sense resistor placed in series with the FET source will produce an erroneous sense voltage. What is needed is control circuitry for driving a BJT-FET cascode while negating the effect of the BJT base current in the current sense resistor.